1. Field of the Invention
The present invention relates to a method of initializing hard bias of a magnetic read head and, more particularly, to enhancing hard bias layers which longitudinally bias and stabilize the free layer of a read sensor.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic field signals from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic field signals from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90° to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. Two leads, designated R+ and R−, are connected to two ends of the spin valve sensor for conducting a sense current therethrough. The magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of external magnetic field signals.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimized and when the magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos θ, where θ is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonmagnetic electrically nonconductive first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
First and second hard bias layers typically abut first and second side surfaces of the spin valve sensor for longitudinally biasing the free layer. Longitudinal biasing promotes a single magnetic domain state of the free layer so that its operation is more predictable. Without proper biasing the magnetic moment of the free layer, which should be parallel to the ABS and parallel to the major thin film planes of the sensor, may not return to its parallel quiescent position after being rotated by a field signal from the rotating magnetic disk. Furthermore, a less than sufficient longitudinal biasing will encourage the formation of edge domains and their subsequent movements resulting in output signal instabilities.
The read head in a hard disk drive (HDD) is first mounted on a head gimbal assembly (HGA) which is then swaged onto a head stack assembly (HSA). An HSA may contain one or more HGAs. Two types of HGAs, UP and DOWN or DN, are usually required to read data from a single rotating magnetic media. The UP HGA typically refers to the HGA that has its ABS facing up to the bottom of the magnetic media while the DN HGA is the one with its ABS facing down the top surface of the media when it is in a horizontal position. It is not unusual to have read amplitude instability failure ratios of two or more to one between UP and DN HGAs. The UP and DN HGAs generally come from separate wafers because their structures are generally mirror images of each other. For ease of identification, the wafers corresponding to the UP and DN HGAs will be referred to as the UP and DN wafers, respectively.
In practice, the deposition of the hard bias layers may not be uniform. After forming a plurality of magnetic head assemblies in rows and columns on a wafer the prior art typically initializes the hard bias layers of the magnetic heads by subjecting the wafer to a magnetic field which is oriented parallel to the major thin film planes of the sensor layers in a direction along the length of the free layer of the sensor. Specifically, if the two ends of the sensor are labeled R+ and R−, then the applied magnetic field direction is from R− toward R+. Typically the same magnetic field orientation is then reinforced during a subsequent initialization process at either the HGA level and/or at the HSA level. After mounting a HSA in a magnetic disk drive the sense current is applied in a direction opposite to the magnetic field direction of the hard bias layers. In spite of one or more subsequent re-initializations the read amplitude differences between magnetic heads in the magnetic disk drive can have the aforementioned fallout rate which can cause the drive to have unrecoverable error events.